The present invention relates to high voltage switches and, more particularly, to a multistage spark gap switch that is more compact than those presently known.
A spark gap switch is a high voltage closing switch that is used in pulsed power systems and for protection from transients. A basic spark gap switch consists of two electrodes separated by an insulating medium that can be vacuum or a fluid (gas or liquid). The switch is initially open. It closes upon the formation of a conductive plasma channel (spark) in the insulating medium between the electrodes when a sufficiently high voltage difference is imposed on the electrodes. The conductive channel is formed by a breakdown mechanism that can be driven in one of two ways. The first way (self-breakdown) involves the application of a voltage difference across the electrodes that is higher than the voltage breakdown threshold of the switch, i.e., the voltage at which the electric field in the gap between the electrodes exceeds the electric strength of the fluid, or induces sufficient electron emission from the surfaces of the electrodes into a vacuum. The second way is to induce breakdown at a voltage difference across the electrodes that is below the voltage breakdown threshold. This is done by using a third, trigger electrode to briefly raise the electric field in the gap between the electrodes, or by means such as radiation or a change in insulator pressure that induce degradation of the electric strength of the insulating medium. The simple and robust structure of spark gap switches, and their ability to self-close and to float to high voltages, makes them popular components of devices such as Marx generators.
The repetition rate of the operation of a spark gap switch is limited by the time required for the plasma to recombine and for the heat associated with the discharge to be dissipated so that the insulator returns to its initial electric strength. Therefore, high repetition rate spark gap switches commonly use a fluid (gas or liquid) insulator that flows through the interelectrode discharge gap. Nevertheless, the repetition rate of these spark gap switches usually is only a few tens of hertz. In addition, the high flow rates required by some applications tend to degrade switching reproducibility and introduce complications in overall system design.
FIG. 1A shows a multistage spark gap switch, which is essentially a series of two-electrode spark gap switches connected back to back. Electrodes 10 are held apart by insulating spacers 12 to define discharge gaps 14. The total switch voltage is divided capacitively among discharge gaps 14, allowing discharge gaps 14 to be very small. This gives the multistage structure fast recovery times, enabling operation at repetition rates upwards of several kilohertz. If the insulating medium is a gas, the pressure of the gas can be atmospheric, simplifying the mechanical and operational complexity of the switch. Fluid flow rate can be very low, or fluid flow may not be required at all. The small discharge gap and low pressure allow the switch to operate in a less violent discharge mode, which considerably increases the lifetime of the electrodes and hence of the switch as a whole.
Historically, the multistage spark gap switch, then called a xe2x80x9cquenched spark gapxe2x80x9d, was first used in the 1920s in sparking transmitters because of its fast recovery time and its high repetition rate. Newer transmitter technologies rendered the multistage spark gap switch obsolete in this application, and it has found little application since then. Until recently, high energy, high voltage pulsed power applications required only a low repetition rate, for which a single stage spark gap switch is adequate. The higher repetition rates of the newest high voltage pulsed power generators requires a different switch technology. In principle, the multistage spark gap switch of FIG. 1A is appropriate for these high repetition rates. In practice, however, the length of a typical multistage spark gap switch gives it an undesirably long closing time and an undesirably large inductance in the conducting phase. The extra length of a multistage spark gap switch, compared with an equivalent single stage spark gap switch, also complicates the layout of a generator with many such switches and may increase the size of the generator, thereby degrading its performance in some applications because of the increased weight, larger inductance and longer rise time associated with the larger size.
Thus there is a widely recognized need for, and it would be highly advantageous to have, a multistage spark gap switch design that is shorter than those presently known.
According to the present invention there is provided a spark gap switch, including: (a) a first substantially planar electrode including a discharge portion and a support portion; and (b) a second substantially planar electrode parallel to and spaced apart from the first electrode and including a discharge portion and a support portion, the discharge portions being mutually opposite, and the support portions being mutually staggered.
According to the present invention there is provided a spark gap switch including: (a) a first stack of at least two substantially planar, mutually parallel electrodes, each of the electrodes including: (i) a discharge portion, and (ii) a support portion; the discharge portions of adjacent electrodes being spaced apart and mutually opposite, the support portions of adjacent electrodes being mutually staggered.
According to the present invention there is provided a spark gap switch including: a first stack of at least two substantially planar, mutually parallel electrodes, each of the electrodes including a discharge portion, the discharge portions of adjacent electrodes being mutually opposite and spaced apart by at most about one millimeter.
In the prior art spark gap switch of FIG. 1A, spacers 12 must have a certain minimum length to ensure that the spark is confined to discharge gaps 14 and does not propagate from one electrode 10 to the next along the outer surface of an intervening spacer 12. In practice, this length is several (typically three) times the width of any one discharge gap 14. Electrodes 10 are nonplanar, so that when electrodes 10 are stacked as shown, peripheral gaps 16 that are wider than discharge gaps 14 accommodate spacers 12. The main contribution to the length of these prior art spark gap switches is the width of peripheral gaps 16.
FIG. 1B shows an alternative prior art design of a multistage spark gap switch in which planar electrodes 10xe2x80x2 are separated by insulating spacers 12xe2x80x2. In this design, discharge volumes 14xe2x80x2 are not well-defined and may overlap onto spacers 12xe2x80x2. Therefore, plasma that is produced in discharge volumes 14xe2x80x2 attacks spacers 12xe2x80x2. This leads to frequent surface breakdowns on spacers 12xe2x80x2 that result in irregular operation and short lifetime.
As noted above, to eliminate surface breakdowns along the spacers, the potential surface discharge path along the spacers should be several times longer than the path length of the volume discharge. In the design of FIG. 1B, these path lengths are equal. An improved design in this respect, but still lacking well-defined discharge regions, is shown in FIG. 1C. Planar electrodes 10xe2x80x3 are separated by insulating, spacers 12xe2x80x3 that have corrugated outer surfaces. The corrugations increase the lengths of the spark propagation paths along the outer surfaces of spacers 12xe2x80x3, but in practice, in such a design, the threshold voltage for surface breakdown can not exceed the threshold voltage for volume breakdown. Therefore, even in the design of FIG. 1C, undesired surface discharges occur quite often.
Another disadvantage of the designs of FIGS. 1B and 1C is that it is impractical to produce spacers 12xe2x80x2 for gaps of about 1 millimeter or less, or spacers 12xe2x80x3 for gaps of a few millimeters or less, for three reasons. First, spacers 12xe2x80x2 and 12xe2x80x3 generally are made of ceramic materials, which are too fragile to withstand the mechanical shocks associated with repeated switch discharge. Second, the corrugations of spacers 12xe2x80x3 are less effective in preventing a surface breakdown on such a small scale. Third, small insulators are more sensitive than large insulators to local imperfections such as impurities in the ceramic.
FIG. 2 is a partial perspective view of a simple multistage spark gap switch of the present invention. In FIG. 2 are shown three planar electrodes 20a, 20b and 20c, representative of a stack of parallel planar electrodes 20. Each electrode 20 has a discharge portion 24 and a support portion 26. Discharge portions 24 are positioned opposite each other to define discharge gaps 28 therebetween. Spacers 22, of which only one is shown in FIG. 2, are placed between support portions 26, as in the prior art multistage spark gap switches, to separate electrodes 20. Support portions 26 also serve to conduct heat away from discharge portions 24. Unlike the prior art multistage spark gap switch, support portions 26 are staggered so that spacers 22 separate nonadjacent electrodes 20. So, for example, in FIG. 2, support portion 26b is staggered with respect to support portions 26a and 26c. As a result, the length of the multistage spark gap switch of FIG. 2 is determined only by the thickness of electrodes 20 and the width of discharge gaps 28.
A. Anvari and O. Steinvall, in xe2x80x9cStudy of a 40 kV multistage spark gap operated in air at atmospheric pressurexe2x80x9d, Journal of Physics E, vol. 6 (1973) pp. 1113-1115, presented a multistage spark gap with planar, disc-shaped electrodes separated by annular separators. Their design resembles the design of FIG. 1B. However, to confine the discharge portions of the electrodes to the vicinity of the electrode centers, and in particular to avoid spark propagation along the sides of the spacers, the electrodes were provided with small central holes, and a trigatron arrangement was used to trigger the switch. The electrodes were spaced 4 mm apart. This design is unsuitable for electrodes spaced about one millimeter or less apart because the separators would be too fragile to withstand the shocks associated with repeated discharge. Another disadvantage of this design is its lack of a proper self-closing capability, due to the equal lengths of the potential surface and volume discharge paths, which result in frequent surface discharges along the surfaces of the separators.
In a multistage spark gap switch the present invention, planar electrodes 20 have relatively large, well-defined discharge portions 24 and are spaced relatively close to each other, as compared to prior art multistage spark gap switches. This gives the multistage spark gap switch of the present invention more compactness, a longer lifetime and better reproducibility than the prior art multistage spark gap switches, as well as self-closing capability.
The embodiment of FIG. 2 is not a preferred embodiment of the present invention. It is presented herein only to illustrate the principle of the present invention. Preferred embodiments of the present invention are presented below.